This application claims priority under 35 USC § 119 to Korean Patent Application No. 10-2004-0054493, filed on Jul. 13, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The invention relates to an amplifier, and more particularly, to a class AB amplifier capable of easily controlling the amount of a quiescent current during a static state of the class AB amplifier and controlling an output current in an operating (amplifying) state.
2. Desscription of the Related Art
Push-pull amplifiers made up of CMOS transistors are extensively used in consumer goods, (e.g., mainly in acoustic amplifiers). Amplifiers are most generally categorized into analog amplifiers and digital amplifiers.
Class A amplifiers, class B amplifiers, and class AB amplifiers are analog amplifiers, while class D amplifiers are digital amplifiers. The performance of an acoustic amplifier is commonly measured by its linearity rather than by its (power) efficiency. Therefore, analog linear amplifiers are popular in the field of acoustic amplifiers, even though they are typically the most power efficient. In other words, a typical analog amplifier having excellent linearity has had poor (energy) efficiency.
Until now, when a class A amplifier, a class B amplifier, or a class AB amplifier with excellent linearity has been used a large-output acoustic amplifier, a large power loss has been unavoidable.
In particular, in the case of the class A amplifier, the rate of power loss is typically greater than a maximum rate of amplifier output power. In general, the efficiency rate of the class A amplifier is less than 25%. In class A amplifier, there is always some quiescent power (current) used for bias or reference circuitry that is not delivered to the load.
As an alternative to the class A analog amplifier, a class B push-pull amplifier in which two transistors are combined in the form of an emitter follower to minimize energy loss, has been introduced. However, although the class B push-pull amplifier has higher efficiency, crossover distortion occurs when a signal level is low.
Two transistors installed in the class B amplifier are alternately switched ON or OFF. When a small amount of current flows through the transistors, they are switched on/off at a high speed. However, when a lot of current flows through the transistors, it is difficult to switch them on/off at a high speed. Since no bias current flows through the class B amplifier, the transistors are not switched on/off at a high speed in a region where significant currents flows, which aggravates the total harmonic distortion (THD).
In the case of the class AB amplifier, which is an intermediate type between the class A and B amplifiers, a certain amount of (quiescent) current flows through it even in a static state. The size of the current flowing through the class AB amplifier is far smaller than that of current flowing through the class A amplifier but is much larger than that of current flowing through the class B amplifier. Accordingly, the greater the amount of bias (quiescent) current flowing through the class AB amplifier, the more closely the characteristics of the particular class AB amplifier is to those of the class A amplifier. By contrast, the less the amount of bias (quiescent) current flowing through the class AB amplifier, the more closely the characteristics of a particular class AB amplifier is to those of the class B amplifier.
Accordingly, there is a growing need to control the operation of the class AB amplifier so that a desired amount of current can flow through it in both a static state and an operating state.